Recently, with the advancement of flat panel displays such as liquid crystal display devices, plasma displays and organic EL display devices; IR sensors, optical waveguides, and the like, to optical materials to be used, there have been required not only excellent transparency but high heat resistance and exhibition of necessary birefringence.
For instance, in the case of a protective film of polarizing plate used for a polarizing plate of liquid crystal, an optical material having a lower birefringence, even with identical total light transmittance, is required. On the other hand, in the case of ¼ wavelength plate having function which changes linear polarization polarized by a polarizing plate to circular polarization, an optical material capable of intentionally exhibiting birefringence of required size is necessary.
Further, as a result of larger flat panel displays, a molded article composed of optical material to be used gets larger. However, there is a problem that contrast becomes lower because distribution of birefringence occurs due to bias of external force. In order to make the distribution of birefringence smaller, an optical material having a small change of birefringence due to external force, that is, a smaller absolute value of photoelastic coefficient is required.
Here, in the case where an optical material is a high molecular compound, for instance, when molding by extrusion molding, since the influence of orientation of high molecular chain during melt extrusion molding tends to remain, a molded article having a considerable birefringence is obtained. On the other hand, when molding by cast molding, since the orientation of high molecular chain does not occur, a molded article having no birefringence is obtained. Though in the molded articles obtained by each method, intended birefringence can be produced by drawing, there are problems (i) that, in the case of a material having a property of widely changing birefringence by a little drawing, it is difficult to control its retardation, and (ii) that, in the case of a material which cannot give a necessary birefringence if a draw ratio is not high, the processing is difficult.
Methacryl-based resins represented by methyl methacrylate homopolymer (PMMA) are excellent in transparency and have a low birefringence, and are utilized for various optical uses, but are insufficient in heat resistance. Similarly, styrene-based resins represented by styrene homopolymer (PS) are excellent in transparency, but have a remarkably large birefringence value and are insufficient in heat resistance.
Accordingly, it is required to develop an optical material having a proper retardation suitable for, for instance, manufacturing of a retardation film such as a ¼ wavelength plate, having a smaller change of birefringence by external force and a higher heat resistance (referring to Non-patent Literatures 1 and 2).
As well-known peripheral techniques of the present invention, a three-component copolymer prepared by methyl methacrylate, styrene and maleic anhydride which constitute the thermoplastic acrylic resin of the present invention are disclosed, for example, in Patent Literatures 1 to 4. In Patent Literature 1, there is disclosed the three-component copolymer of methyl methacrylate: 60 to 90% by mass, styrene: 5 to 20% by mass, maleic anhydride: 5 to 20% by mass, and is described that it is preferable from the viewpoints of thermal deformation resistance, weatherability, and the like that a weight ratio (a/b) in the terpolymer where (a) is a content of the repeating unit derived from a vinyl aromatic monomer and (b) is a content of repeating unit of cyclic acid anhydride is not less than 1 to less than 3. In Patent Literature 2, there is disclosed that a sum of the remaining monomer is desirably not more than 1.5% by weight relative to the copolymer, and that the copolymer is colored to yellow when an amount of maleic anhydride remaining is large. In the disclosed examples, all of the copolymers are obtained by bulk polymerization, and the sum of the remaining monomers is not less than 0.5% by weight at a minimum. Particularly, in the comparative example in which the content ratio (a/b) is less than 1, the sum of the remaining monomers is more than 3% by weight, and particularly maleic anhydride particularly tend to remain. On the other hand, in Patent Literature 3, there is disclosed the terpolymer of methyl methacrylate: 45 to 92% by mass, an aromatic vinyl compound: 5 to 30% by mass, maleic anhydride: 3 to 25% by mass. Though there is no description as to the content ratio (a/b) and effects expected from the range, there is described only the three-component copolymer having a ratio (a/b) of larger than 1, i.e. a/b=14/10 in the examples. Similarly, in Patent Literature 4, there is disclosed the three-component copolymer of methyl methacrylate: 70 to 90% by mass, an aromatic vinyl compound: 1 to 25% by mass, and maleic anhydride: 5 to 29% by mass. Though there is no description as to the weight ratio (a/b) of a content (a) of the repeating unit derived from a vinyl aromatic monomer and a content (b) of repeating unit of cyclic acid anhydride and effects expected from the range, there is described only the three-component copolymer having a ratio (a/b) of larger than 1, i.e. a/b=15/12 in the examples.
On the other hand, for example, Patent Literatures 5 to 8 disclose peripheral techniques relating to the four-component copolymer having benzyl methacrylate as the fourth monomer, of the more preferable thermoplastic acrylic resin of the present invention. Patent Literature 5 discloses a copolymer of methyl methacrylate with any one of the copolymerizable monomers such as styrene, benzyl methacrylate, and maleic anhydride. However the description directs to one of compositions which constitute a thermoplastic antistatic laminated article, and there is no description about optical properties at all. Moreover, there are no examples of the four-component copolymer corresponding to the present invention.
In Patent Literature 6, there is a description as to a copolymer containing styrenes, maleic anhydrides and methacrylate esters. Specifically, there is described that, as the metacrylate esters, methyl metacrylate and benzyl methacrylate may be copolymerized. However, there is not described an example as to the four-component copolymer composed of methyl methacrylate, styrene, benzyl methacrylate and maleic anhydride. Further there is described that a lower alkyl ester is preferable as the methacrylate esters, and there is no suggestion as to the repeating unit derived from methacrylate monomer having an aromatic group of the present invention. In addition, there is described that a preferred resin is a copolymer in which a part or the whole of maleic anhydride which is the structural unit is subjected to hydrolysis.
In Patent Literature 7, there is a description as to a copolymer containing, as a main component, a monomer selected from styrenes, maleic anhydrides and methacrylate esters. Though the monomers of methacrylate esters include methyl methacrylate and benzyl methacrylate, there are specific descriptions as to a blend of a copolymer of styrenes and methacrylate esters and a copolymer of maleic anhydrides and methacrylate esters, and a blend of a copolymer of styrenes and maleic anhydrides and a copolymer of maleic anhydrides and methacrylate esters, and there is no mention about effects obtained by copolymerizing three or more monomers at the same time. Particularly, there is no description of any example as to the four-component copolymer of the present invention. There is described that the methacrylate in the copolymer is preferably an ester of a lower alkyl ester, and there is no suggestion as to the repeating unit derived from methacrylate monomer having an aromatic group of the present invention. In addition, there is described that a preferred resin is a copolymer in which a part or the whole of maleic anhydride which is the structural unit is subjected to hydrolysis.
Further, in Patent Literature 8, there is a description as to a copolymer of maleic anhydride and acrylate. Specifically, there is described that methyl(meth)acrylate and benzyl(meth)acrylate may be used at the same time as the acrylate monomer in the copolymer, and that styrenes may be copolymerized as other monomers insofar as heat resistance is not impaired. However, there is no description of any example as to the four-component copolymer of the present invention.